NANO Ph.D. Seminar Notice: Fabrication of atomic force microscope probes with high aspect ratio silicon tips, silicon/silicon nitride cantilevers and stair-shaped handles

Wednesday, July 10, 2024 9:00 am - 10:30 am EDT (GMT -04:00)

Candidate: Aixi Pan

Title: Fabrication of atomic force microscope probes with high aspect ratio silicon tips, silicon/silicon nitride cantilevers and stair-shaped handles

Date: July 10, 2024

Time: 9:00 AM


Supervisor(s): Cui, Bo (Adjunct) - Ban, Dayan


The atomic force microscope (AFM) is a versatile tool with promising applications in biomedical detection, optical spectroscopy, and material characterization. However, its widespread utilization faces challenges due to limitations in conventional fabrication methods of commercially available probes. Specifically, the standard tips may not meet the requirements for scanning deep or narrow structures accurately, and the rectangular handle design can block a portion of the reflected laser signal, leading to inconsistent feedback. Overcoming these limitations is crucial for expanding the utility and unlocking the full potential of AFM in diverse scientific and technological applications. In this thesis, we propose innovative strategies to enhance the scanning performance of AFM probes with high aspect ratio (HAR) tips and stair-shaped handles.

Chapter 3 explores four distinct silicon (Si) AFM tip fabrication methods, each offering unique advantages and contributions to the field. The first method (Section 3.1) integrates non-switching pseudo-Bosch etching with wet sharpening techniques to achieve an exceptional aspect ratio of 1:135, marking a significant advancement in tip fabrication. The second method (Section 3.2) introduces an innovative approach utilizing tapered silicon oxide (SiO2) masks to fabricate Si nanocones with extraordinary apex measuring just 3.54 nm. The third method (Section 3.3) explores a novel two-step cryogenic etching process to yield a controllable tip profile with a slight taper angle of 2.2°. The fourth method (Section 3.4) combines the Bosch process with the pseudo-Bosch process, incorporating periodic oxygen (O2) shrinking steps. This approach achieves a remarkable tip apex sharpness of 20 nm, pushing the boundaries of nanofabrication capabilities.

Chapter 4 details the fabrication of a stair-shaped Si handle to mitigate laser blocking. Two techniques are described: one leveraging the loading effect and RIE-lag to attain stage heights of 71  m, 151  m, 168  m, and 287  m, while the other employs pseudo-grayscale lithography with a titanium (Ti) mask, yielding final stages of 52  m, 83  m, 161  m, and 211  m. Both these methods are applicable for practical AFM probe fabrication without laser blocking.

Chapter 5 delves into the mass fabrication of all-Si HAR AFM probes, merging tips fabricated by the O2 shrinking method with handles fabricated using the loading effect and RIE-lag. Furthermore, Chapter 6 explores the adoption of silicon nitride (Si3N4) as an alternative to Si for cantilevers. Amorphous low-pressure chemical vapor deposition (LPCVD) Si3N4 offers benefits such as low spring constants and precise deposition control, resulting in thin and low-spring constant cantilevers. This configuration minimizes damage to the sample and tip, making it ideal for delicate samples. By combining a Si3N4 cantilever with a Si tip, the probe capitalizes on both tip apex and thin cantilever advantages, facilitating accurate AFM imaging with high resolution while preventing false images on fragile samples.